Added support for simple addition of two values.
Added support for simple subtraction of two values.
Refactored to improve naming and remove duplication of 'if' and 'split' statements - most slight cheating as changed Added/Subtractor classes directly, but could have just as easily added new classes, changed factory to use and deleted old classes...
Added support for simple multiplication of two values.
Added support for simple division of two values.
Added final test.